The invention relates to a piston engine comprising a piston, which is movable in a reciprocating manner in a cylinder, displaces a gaseous medium and is journalled in a radial direction with respect to the direction of movement of the piston by means of at least one dynamic groove bearing.
The invention further relates to a cryo-cooler provided with a piston engine of the kind mentioned.
In a piston engine known from European Patent Application EP-A1-0223288 corresponding to U.S. Pat. application Ser. No. 928,063, (PHN 11538) of the kind mentioned in the opening paragraph, the piston rotatable in the cylinder is provided with a dynamic groove bearing. In a large number of cases, it is objectionable to subject the piston engine to a combined rotary and translatory movement. Thus, it is no longer possible to provide by means of a dynamic groove bearing on the outer side of the piston a radial journalling of the translating piston. A rotation of the piston is impossible, for example, when the piston is coupled to the coil of a linear electric driving motor. The electrical connections required are capable of withstanding only a limited rotation. It further appears increasingly more difficult to bring the tolerances of piston and cylinder (radial gap width) into conformity with the dimensions in radial direction of an optimally operating dynamic groove bearing. This is the more the case as the piston engine is a so-called cryo-cooler, in which the piston is constituted by a free displacer. The requirements imposed in such a cryo-cooler on the radial gap width in connection with variations in the phase difference between the translatory movement of the free displacer and the translatory movement of the piston are generally of a quite different nature from the requirements with respect to an optimally operating groove bearing and the attainable manufacturing tolerances. The comparatively great temperature differences over the displacer also influence the radial gap width.